By spotting that someone is slowing several vehicles ahead, you significantly increase your chances of averting a multi-car shunt. US research suggests this sort of crash has been reduced by more than half since high-level lamps were made compulsory there in 1985.

The lights are now included on new cars, but if your ageing runabout doesn't have one, you should seriously consider an aftermarket fit.

The choice has shrunk since our last test in 2000, largely because there are fewer cars on sale that don't include them as standard. But there are still some out there, and here's our guide to the best.

When buying a high-level lamp, ensure that it carries the 'E' mark.

This is required under EU law - if the device hasn't got the mark, you can't use it as a third brake light.

The most simple products rely on a single 21W bulb, but if this blows, the lamp is out of action.

Those with more, lower-wattage, bulbs don't have that problem, but are more expensive. Make sure you add up the bulb wattages.

LED (light-emitting diode) lamps are much better all round - typically, an LED illuminates 0.2 seconds quicker than a bulb.

Although that doesn't sound much, it could mean five metres additional reaction distance at 70mph.

As LEDs are smaller, you can get more into the same space and they can't blow like a bulb, so never need replacing.

Size is important, too. A wider lamp attracts more attention from following drivers, but will reduce your vision.

You also need to consider the aesthetics of a wide lamp in a small car - it could look plain silly.

We were looking for good build quality, a versatile design and clear instructions, preferably with diagrams included.

Next, we checked if the light would be flared back on to the rear screen and become a potentially dangerous distraction in the rear view mirror.

This is not easy to do, but it is a vital factor in choosing the right lamp.

Finally, we balanced size, wattage output and design against price to come up with our winners and losers.

Ring RL 099

Rating: ****

Price: &#xA3;19.99

LEDs/fitment: 20/screen, shelf

There are only 20 LEDs in this model - it is 70mm narrower than the 28-LED Ring - but it was very effective regardless. For screen-mounting, the angle is 25 to 85 degrees. When fitted this way, it sealed right round the mask, so there was no flare. Fitting the two-joint stand could allow some. A reasonable price made it more desirable.

Ring RL 097

Rating: *****

Price: &#xA3;14.99

LEDs/fitment: 28/screen

The joint-lowest priced LED model alongside the Ring RL 096, which also had 28 LEDs - although the latter's design is very different. It could only be stuck to the screen, but the ball joints in the feet allowed the lens to swivel to most angles.

It is our Recommended pick due to its high performance and minimal flare from the very well hooded lens.

Ring RL 096 *****

Price: &#xA3;14.99

LEDs/fitment: 28/screen, shelf

A modern-looking lamp that manages to be svelte despite its 28 LEDs. It came with a good set of instructions and could be fitted either on a shelf or screen to suit a particular vehicle.

The lens had no exterior shielding, so flare was present.

However, the low price of &#xA3;14.99 makes this our LED Best Buy.

Cosmic Slim Fit Hi-Mount C651

Price: &#xA3;19.99 ****

LEDs/fitment: 36/screen, shelf

This Cosmic model bears more than a passing resemblance to the Ring RL 096, although it was around 20mm narrower.

Despite this, it managed to include more LEDs and the clear lens is certainly much more stylish than the red one.

It can be mounted wherever you want, although there was no exterior shielding, which led to some flare on the screen.

Cosmic Slim Fit LED C611

Rating: ****

Price: &#xA3;19.99

LEDs/fitment: 36/screen, shelf

Living up to its name perfectly, this model is incredibly thin, and weighs virtually nothing at all, despite cramming in 36 LEDs.

The clear lens was probably taken from the C651, but the fully adjustable feet at either end meant it could be mounted on the screen or the shelf. Good instructions and versatility offset the lack of lens shielding.

Hella Safety Brake Light 2DA 007 592 101

Rating: ***

Price: &#xA3;54.97

LEDs/fitment: 16/screen

Build quality here was the best on test - as you'd expect from Hella.

Once it's in position on the screen, there is very little chance of any flare in the mirror. Instructions were comprehensive and the clear lens desirable, but only 16 LEDs are fitted and, with no shelf-mount option, the price is far too high for this competitive market.

Street Wize JS196

Rating: ***

Price: &#xA3;20.99

LEDs/fitment: 28/screen, shelf

If you think this looks familiar, there's a reason why - it is clearly based on the Cosmic C611.

However, this model has to make do with 28 LEDs, as opposed to the Cosmic's 36, and a red lens.

It was equally light and easy to fit, and had good instructions, but the lack of shielding was a significant downside.

Street Wize JS198 Stealth

Rating: ***

Price: &#xA3;15.99

LEDs/fitment: 12/Various

An unusual lamp that, at around 150mm long, is just big enough to hold in your hand. Self-sticky pads are fitted to the end feet and it can be fitted to any suitable surface, inside or out - or even on a motorcycle/scooter. In many ways, it is impressive and versatile, but it was virtually impossible to fit inside a rear window due to the lack of adjustment.

Verdict:

For a bulb-based lamp, we would opt for Ring's RL 087.

It's powerful, reasonable and is well shielded.

In our last LED test, the Cosmic cleaned up, but Ring's prices swung it this time, with the firm taking first and second.